Sorry, can't post the code (it's proprietary), and not necessary to answer my question. My question is basically, "What the heck does this do?" Yes, I think I know; it reads the ADCH and ADCL registers. But I can't be absolutely sure unless I understand where it comes from.
The #define statement you listed makes sense. But where is it?
This is for an Arduino Nano. (Processor is a 328P).